Porous ceramics have been used in a variety of applications such as filters and catalyst substrates. Recently more stringent diesel particle emission standards have been promulgated in the United States and Europe. To achieve these more stringent particle emission standards, diesel particulate filters are expected to be necessary.
These particulate filters will have to meet multiple contradictory exacting requirements. For example, the filter must have sufficient porosity (generally greater than 55 percent porosity) while still retaining most of the emitted micrometer sized diesel particulates (generally greater than 90 percent capture of the emitted particulates). The filter must also be permeable enough so that excessive back pressure does not occur too quickly, while still being able to be loaded with a great amount of soot before being regenerated. The filter must withstand the corrosive exhaust environment for long periods of time. The filter must have an initial strength to be placed into a container attached to the exhaust system.
Most importantly, the filter must be able to withstand thermal shock from rapid heating and cooling, for example, due to being exposed to a puddle at operating temperature and burning off the soot to regenerate the filter. From these stringent criteria, ceramic filters have been the choice of material to develop a diesel particulate filter.
Early on, ceramic filters of sintered cordierite were explored as a possible diesel particulate filter. Cordierite was explored because of its low cost and use as a three-way catalyst support in automotive exhaust systems. Unfortunately, cordierite has not displayed the capability of high porosity, high permeability and high soot loading in conjunction with the thermal shock resistance and abusive environment experienced during regeneration and operation.
More recently, silicon carbide has become of interest as a filter material because of its high strength and high melting point compared to cordierite. However, silicon carbide suffers, for example, from having to be sintered at high temperature using expensive fine silicon carbide powder. Even with this improved strength, SiC diesel particulate filters have been made by cementing segments of SiC honeycombs together to manage the thermal stresses by preferentially causing cracking in the cement as opposed to the SiC honeycombs themselves. This cementing leads to increased complexity and is at best an interim solution.
To lessen the thermal stresses associated with the burning of the soot in the filter, soot catalysts and engine management protocols have been used to lessen the temperature where the soot is combusted. Even so SiC filters still have to be made by cementing segments of SiC honeycombs together to manage the thermal stresses.
What is needed is a filter that has increased strength while at least having the same or improved thermal shock resistance. Thermal shock resistance is proportional to the strength of the body and inversely proportional to the elastic moduli (i.e., stiffness) and the coefficient of expansion. Unfortunately, simply increasing the strength of a porous body has typically increased the density (lowered the porosity) and/or increased the elastic moduli resulting in, many instances, reductions or no improvement of the thermal shock resistance.
Accordingly, it would be desirable to provide a porous ceramic body and method to form such a body that has, for example, increased strength while having the same or improved thermal shock resistance without substantially diminishing the porosity of the filter.